According to a report in the Daily Telegraph, Apax told its investors in March that it had now written down the investment in the business to "zero".
Emap, which publishes Draper's Record, Nursing Times, Retail Week and Broadcast, has been hard hit by the rapid decline in the advertising market.
At the end of last year, it abolished subscription charges for many of its websites, including Health Service Journal, Retail Week and Drapers.
This could now well return in the future as many publishers reconsider their content models.
Rivals such as Incisive Media and Haymarket, which publishes Brand Republic, have also been suffering.
Earlier this week, Incisive Media closed down the long standing Personal Computer World Magazine as sales and advertising evaporated.
However, despite the write-down Apax stressed that it did not reflect the real value of Emap, which despite steep falls in ad revenues is performing well in its key markets.
It added that an improvement in the markets could well see the value upgraded again.
A spokesman for Guardian Media Group told the Telegraph that it was "very happy with Emap's performance, both in operating terms and as an investment for GMG".
Chief executive David Gilbertson, who was appointed in March 2008 and joined from Informa, has implemented cuts in the face of the downturn including axing 40 jobs among its 400-strong workforce.